Certain embodiments of the present invention are directed to integrated circuits. More particularly, some embodiments of the invention provide a system and method for dimming control using TRIAC dimmers. Merely by way of example, some embodiments of the invention have been applied to driving light emitting diodes (LEDs). But it would be recognized that the invention has a much broader range of applicability.
Lighting systems including light emitting diodes (LEDs) often use a conventional light dimmer (e.g., wall mounted) that includes a Triode for Alternating Current (TRIAC) to adjust the brightness of LEDs. A TRIAC is bidirectional and currents can flow through a TRIAC in either direction (e.g., into the TRIAC or out of the TRIAC). A TRIAC can be triggered by a gate current (e.g., flowing in either direction) which is often generated by applying a voltage (e.g., a positive voltage or a negative voltage) to a gate electrode of a TRIAC. Once triggered, the TRIAC continues to conduct a current until the current drops below a certain threshold (e.g., a holding current). For example, a TRIAC dimmer is a dimmer (e.g., a light dimmer) that includes a Triode for Alternating Current.
FIG. 1 is a simplified diagram showing a conventional LED lighting system using a TRIAC dimmer. For example, the lighting system 100 implements a primary side regulation scheme and a flyback structure with single-stage power-factor-correction (PFC) for driving one or more LEDs 172. The system 100 includes a controller 102, an AC supply component 122, a TRIAC dimmer 118, a full wave rectifying bridge 124, capacitors 126, 136, 140, 150 and 170, resistors 128, 130, 134, 138, 146, 148, 154 and 156, power switches 132 and 152, diodes 142, 144 and 168, and a transformer including a primary winding 162, a secondary winding 164, and an auxiliary winding 166. The controller 102 includes terminals 104, 106, 108, 110, 112, 114, 116 and 120. For example, the power switch 132 and the power switch 152 are transistors. In another example, a TRIAC dimmer 118 is a dimmer that includes a Triode for Alternating Current (TRIAC).
As shown in FIG. 1, the TRIAC dimmer 118 processes an AC input signal 121 from the AC supply component 122, and generates a voltage signal 123 which is processed by the full wave rectifying bridge 124 in order to generate a voltage signal 174 (e.g., Vbulk). The TRIAC dimmer 118 is associated with a dimming period including an on period and an off period. During an on period of the TRIAC dimmer 118, the voltage signal 174 is approximately equal to the AC input signal 121 in magnitude. During an off period of the TRIAC dimmer 118, the voltage signal 174 has a low magnitude (e.g., corresponding to a logic low level). The capacitor 150 (e.g., C1) is charged in response to the voltage signal 174 (e.g., Vbulk) through the resistor 138 (e.g., R3), and a voltage signal 176 is generated at the terminal 112 (e.g., terminal VCC). If the voltage signal 176 exceeds an under-voltage-lock-out (UVLO) threshold voltage, the controller 102 is activated, and outputs a modulation signal 178 (e.g., a pulse-width-modulation (PWM) signal) through the terminal 116 (e.g., terminal GATE) in order to close (e.g., to turn on) or open (e.g., to turn off) the switch 152 (e.g., M2) for normal operation of the system 100. A voltage divider circuit including the resistor 130 (e.g., R2) and the resistor 134 (e.g., R4) generates a voltage signal 179 based on at least information associated with the voltage signal 174 (e.g., Vbulk). The controller 102 detects the signal 179 at the terminal 106 (e.g., terminal VS) in order to affect the power factor and determine the status of the TRIAC dimmer.
When the controller 102 changes the modulation signal 178 to close (e.g., to turn on) the switch 152 (e.g., M2), a primary current 180 flows through the primary winding 162, and a current-sensing signal 188 is generated through the resistor 154 (e.g., RS). The controller 102 detects the current-sensing signal 188 at the terminal 120 (e.g., terminal CS). For example, the peak values of the current-sensing signal 188 affect the signal 178 to open (e.g., to turn off) the switch 152 in each cycle. An auxiliary current 182 flows through the auxiliary winding 166 to charge the capacitor 150 (e.g., C1), and a voltage signal 184 is generated at the auxiliary winding 166. A voltage divider circuit including the resistor 146 (e.g., R5) and the resistor 148 (e.g., R6) generates a voltage signal 186 based on at least information associated with the voltage signal 184. The controller 102 receives the signal 186 at the terminal 114 (e.g., terminal ZCD) in order to detect the end of a demagnetization process associated with the transformer including the secondary winding 164. In addition, the capacitor 170 is used to maintain output voltage for stable current output to the LEDs 172. During the on period of the TRIAC dimmer 118, the power switch 132 (e.g., MD is open (e.g., off). During the off period of the TRIAC dimmer 118, the power switch 132 is closed (e.g., on) to provide a bleeding current in order for the TRIAC dimmer 118 to operate normally.
FIG. 2 shows a simplified diagram showing a relationship of brightness of the LEDs 172 as a function of the output current 198. The waveform 1302 represents the brightness of the LEDs 172 as a function of the output current 198. The brightness of the LEDs 172 does not change linearly with the output current 198.
FIG. 3 shows a simplified timing diagram for the conventional LED lighting system 100. The waveform 1502 represents the AC input signal 121 as a function of time, the waveform 1504 represents a voltage signal 123 in ideality as a function of time, and the waveform 1506 represents the voltage signal 174 in ideality as a function of time.
As shown in FIG. 3, the AC input signal 121 has a period of 2π (e.g., as shown by the waveform 1502). Ideally, the TRIAC dimmer 118 processes positive values and negative values in the AC input signal 121 the same to generate the voltage signal 123 (e.g., as shown by the waveform 1504). For example, during a part of a period (e.g., corresponding to a phase angle ϕ), the voltage signal 123 follows the AC input signal 121 (e.g., approximately equal to the AC input signal 121 in magnitude), as shown by the waveform 1504. During the other part of the period, the voltage signal 123 has a low magnitude (e.g., zero). The phase angle ϕ is in a range of 0 to π. The full wave rectifying bridge 124 processes the voltage signal 123 and generates the voltage signal 174 with a period of π. The voltage signal 174 (e.g., Vbulk) has a same waveform during each on time period of the TRIAC dimmer 118 (e.g., as shown by the waveform 1506).
As an example, in order for the TRIAC dimmer 118 to operate normally, a bleeding current with a sufficient magnitude needs to be provided to flow through the TRIAC dimmer 118. As another example, if the phase angle ϕ is smaller than a phase-angle threshold (e.g., ϕ0), the voltage signal 123 has a smaller magnitude and the magnitude of the bleeding current becomes smaller than a bleeding current threshold. As yet another example, if the magnitude of the bleeding current becomes smaller than the bleeding current threshold, the TRIAC dimmer 118 cannot operate normally. As yet another example, if the magnitude of the bleeding current becomes smaller than the bleeding current threshold, the TRIAC dimmer 118 is turned off, causing a rapid decrease of the current flowing through the LEDs 172. For example, the TRIAC dimmer 118 is incapable of generating, with the rectifying bridge 124, a pulse associated with a pulse width smaller than the phase-angle threshold (e.g., ϕ0). In another example, the TRIAC dimmer 118 is capable of generating, with the rectifying bridge 124, a pulse associated with a pulse width larger than the phase-angle threshold (e.g., ϕ0).
FIG. 4 is a simplified conventional diagram of the controller 102 as shown in FIG. 1. The controller 102 includes a comparator 202, an error amplifier including a comparator 204 and a switch 205, a logic control component 206, a gate drive component 208, a signal generator 210 (e.g., a PWM signal generator), a multiplier 212, and a current regulation component 214. For example, the signal generator 210 is configured to generate one or more pulse-width-modulation signals. In another example, the signal generator 210 includes a comparator. The current regulation component 214 includes a current-sensing component 298 and another error amplifier 296.
As shown in FIG. 1 and FIG. 4, the comparator 204 receives the signal 179 and a threshold signal 226 in order to detect the status of the TRIAC dimmer 118, and outputs a dimming signal 228. The switch 205 is closed or open in response to the dimming signal 228 to affect the output current in order to adjust the brightness of the LEDs 172 (e.g., to adjust the brightness of the LEDs 172 linearly as a function of the output current). The current regulation component 214 receives the current sensing signal 188 at the terminal 120 (e.g., terminal CS) to detect the peak values of the primary current 180, and integrates the peak values of the primary current 180 over a demagnetization period associated with the transformer including the primary winding 162 and the secondary winding 164. Specifically, the current-sensing component 298 receives the current sensing signal 188 and outputs a signal 294 to the error amplifier 296. The error amplifier 296 also receives a reference signal 292 and outputs a processed signal 216 to the multiplier 212 which also receives the voltage signal 179 from the terminal 106 (e.g., terminal VS) and generates an output signal 218.
The signal generator 210 receives the current sensing signal 188 and the output signal 218 and generates a signal 220. During an operating cycle, if the modulation signal 178 is at a logic high level and the switch 152 is closed (e.g., turned on), the primary current 180 flowing through the switch 152 increases in magnitude. In response the current sensing signal 188 increases in magnitude. If the signal 188 becomes larger than the output signal 218 in magnitude, the signal generator 210 changes the signal 220 and the controller 102 changes the signal 178 from the logic high level to a logic low level to open (e.g., to turn off) the switch 152. When the switch 152 is opened (e.g., turned off), the transformer including the primary winding 162 and the secondary winding 164 begins the demagnetization process.
The comparator 202 receives the signal 186 at the terminal 114 (e.g., terminal ZCD) and a threshold signal 222 to detect whether the demagnetization process has completed. If the demagnetization process is determined to be completed, the comparator 202 outputs a signal 224 in order to change the signal 178 to the logic high level. During the off period of the TRIAC dimmer 118, the logic control component 206 outputs a signal 230 to close (e.g., to turn on) the switch 132 (e.g., M1) in order to provide a bleeding current for the normal operation of the TRIAC dimmer 118.
During the off period of the TRIAC dimmer 118, an average value of an output current 198 is determined as follows:
                                          I            0                    _                =                              1            2                    ×          N          ×                                    V              ref_ea                                      R              s                                                          (                  Equation          ⁢                                          ⁢          1                )            where N represents a turns ratio between the primary winding 162 and the secondary winding 164, Vref_ea represents the reference signal 292, and Rs represents a resistance of the resistor 154. When the TRIAC dimmer 118 is turned on and off to perform dimming control, an average value of the output current 198 is determined as follows:
                                          I            0                    _                =                              ϕ            π                    ×                      1            2                    ×          N          ×                                    V              ref_ea                                      R              s                                                          (                  Equation          ⁢                                          ⁢          2                )            where ϕ represents a phase angle associated with the TRIAC dimmer 118.
The system 100 has some disadvantages, such as flickering of the LEDs 172 under certain circumstances. Hence it is highly desirable to improve the techniques of dimming control.